Electron Acceleration by Whistler Mode Chorus Waves in Jupiter's Radiation Belts

Synchrotron radiation observed at Jupiter indicates a population of relativistic electrons near 1.4 Rj with energies up to 50 MeV. How such electrons are accelerated is a major unresolved question. Even the widely accepted theory of betatron acceleration as a result of inward radial diffusion requires a source of 2 MeV electrons near 15 Rj, the origin of which has not been identified. Here we show that very low frequency waves observed by the Galileo spacecraft, known as whistler mode chorus, resonate with 0.1 - 10 MeV electrons violating the first adiabatic invariant and results in net electron acceleration. Wave acceleration is most efficient outside the orbit of Io due to the variation in magnetic field and plasma density. Outside Io plasma interchange instabilities also provide a natural source for the waves. Assuming the wave power observed near 10Rj by Galileo is not exceeded at larger distances wave acceleration should be most effective near 15Rj. Wave acceleration also produces anisotropic pitch angle distributions peaked near 90 degrees consistent with observations. We suggest that chorus waves accelerate electrons between 0.1 - 10 MeV outside the orbit of Io which are then transported inwards by radial diffusion and accelerated up to 50 MeV via betatron acceleration.